1. Technical Field
The present invention relates to a power generating device that derives electric energy from electric charge which results when a piezoelectric material such as a piezoelectric device receives an external force.
2. Related Art
When a piezoelectric material such as lead zirconate titanate (PZT), crystal (SiO2), or zinc oxide (ZnO) has an external force applied thereto, electric polarization is caused inside the material, and positive and negative electric charge appears on the surface. Such a phenomenon is called a piezoelectric effect. A method of generating power is proposed in which, by using such a property of the piezoelectric material, a cantilever is vibrated so as to repeatedly weight the piezoelectric material, and electric charge generated on the surface of the piezoelectric material is derived as electricity.
For example, by vibrating a metal cantilever which has a spindle arranged at its tip end and to which a thin plate of a piezoelectric material is attached, positive and negative electric charges which are alternately generated in the piezoelectric material in accordance with the vibration can be captured, thereby generating an AC current. A technique is proposed in which such an AC current is rectified by a diode, then is stored in a capacitor, and is captured as electric power (JP-A-7-107752). In addition, a technique is proposed in which a DC current is acquired without incurring a voltage loss in a diode by closing the contact point only while positive charge is generated in the piezoelectric device (JP-A-2005-312269). By using such techniques, the power generating device can be miniaturized. Accordingly, for example, an application of building the power generating device into a small-sized electronic component instead of a battery cell or the like is expected.
However, according to such proposed general techniques, there is a problem that an acquired voltage is limited to the voltage level up to a voltage that is generated in accordance with the electric polarization of the piezoelectric material. Accordingly, in many cases, a booster circuit is necessary in addition to a power generating circuit that derives electricity from the piezoelectric material, and thus there is a problem that it is difficult to decrease the size of the power generating device.